Multimode (MM) commercially available intra-fiber combiners are nowadays very common devices. If operated at their ultimate performance, they can approach output brightness preservation, in parallel with nearly perfect power transmission. However, only poor (i.e. high value of) spatial beam-parameter-product (BPP), a quantitative measure of brightness level, can be obtained, and therefore these devices are not suited for directed radiation applications. (See, for example, U.S. Pat. No. 7,046,875 “Optical coupler comprising multimode fibers and methods of making the same” and U.S. Pat. No. 6,434,302 “Optical couplers for multimode fibers”.)
Recently, strictly or nearly strictly SM (single-mode) combiners were demonstrated in a power range of few watts to several kw, either under incoherent as well as coherent operation regimes (Y. Shamir et-al, JOSA-B, V. 27, N. 12, 2669-2676 (2010), Y. Shamir et-al, Proc. SPIE, Photonics West, DOI: 10.1117/12.841720 SF. CA. (2010), Y. Shamir et-al, Optics Letters, V. 36, N. 15, 2784-2876 (2011), Y. Shamir et-al, OASIS, SPIE meeting on Optical Engineering and Science, TLV. IL (2011), Y. Shamir et-al, Optics Letters, V. 39 N. 7 1412-1414 (2012)). On either of the operation regimes (incoherent or coherent), these devices are capable of producing as well as delivering the lowest obtainable BPP for side by side combining, basically as a result of keeping adiabatic mode propagation within itself. High power operation was successful since LMA (large-mode-area) fibers and large MM delivery fibers were used, thereby reducing power density.
Guiding the combined signal further away from the combining element and yet preserving the BPP obtained by the combiner, has been a long-standing-issue in the field of high power combiner applications. A suggested solution has been previously proposed by the inventors, stating that if a specialty designed core, which can be specifically a GI (graded-index) core, and more specifically, a PI (parabolic-index) core fiber, is used, the spatial properties, i.e. BPP, at any cross-section, can be preserved. Several demonstrations confirmed these assumptions up to a few Kw. However under extreme power levels of multi Kw, new barriers, in the form of thermal distortions, nonlinearities and fiber silica damages, are the next issue to be addressed. A problem is that multi-Kw optical beams are destructive to fibers and related components under long propagation in narrow spot size.
A method for achieving this goal is to combine several techniques in parallel. A straightforward approach is just to fabricate wider devices, and obtain larger beam spot diameter correspondingly; this is possible, although at the expense of BQ (beam quality), a cost that cannot be paid in some applications. Second direction is to use the latter with combinations of additional factors: lower dopant concentration, different dopant materials, as well as using larger delivery core, so as to battle the negative effects in parallel. In what follows we show guidelines for resolving the high power challenge.